LEDs are a type of solid state semiconductor devices capable of converting electrical power into visible light. The application of LEDs primarily comprises LED backlight, LED illumination, and LED display. Currently, cold cathode fluorescent lamps (“CCFLs”) are used as a light source in most liquid crystal displays (“LCDs”). The CCFLs have a relatively short lifetime and are not environmentally friendly, and LEDs have been steadily replacing CCFLs as backlight sources.
An LED backlight system commonly includes strings of serially coupled LEDs and a corresponding LED driver circuit. A relative high power supply voltage is typically required to drive the serially coupled LEDs, and the LED driver circuit has to tolerate high voltage stress. The LED backlight system may include over-voltage protection for preventing damage to the LEDs during open circuit. On the other hand, the over-voltage protection also limits the number of LEDs in each of the LED strings. As a result, a plurality of LED strings are coupled in parallel if large numbers of LEDs are needed for luminescence. However, the resistance of a particular LED may be different than others, and thus a current flowing through a particular LED may be different from that through other LEDs. Such current differential may lead to variable luminance of the LED backlight system and/or even damage some of the LEDs.